1. Field of the Invention
The invention relates to encapsulant compositions used for packaging electronic components and devices. More particularly the invention relates to a thermomechanical-shock-resistant, cured composition for solventless, hydrophobic resin encapsulation of electronic components comprising a non-silicone oligomer including a flexible hydrocarbon backbone having reactive functionality. The composition includes up to about 40% of an adhesion promoter by weight of the total composition.
2. Description of the Related Art
Advances in electronic circuit design have increased the number of electronic components occupying the area available on a printed circuit board. Evidence of such increases can be appreciated by considering the mounting numbers and increased versatility of electronic devices such as personal digital assistants, cellular phones, portable compact disc players, personal computers, lap-top computers and other equipment relying on electronic devices for effective performance. Progressive board space consumption, by semiconductor integrated circuit components, raises problems with a large quantity of very small contacting locations that require interconnection to outside or external circuitry. An interconnection structure, typically positioned around a space occupied by e.g. a semiconductor chip, may be referred to as a lead frame. Lead frame technology has led to tape automated bonding (TAB) technology, referring to connections to a conductor pattern placed on a tape filament. Associated with lead frame technology is another connection scheme, known as wire bonding, wherein segments are bonded to the contact or pad on a chip at one end and to an external conductor at the other end. A problem with wire bonding technology occurs when wires break during injection molding of encapsulant to surround a semiconductor device. Individual connecting wires, of the wire bonding pattern, may snap upon impact of the advancing wave of injected encapsulant material. Therefore protection of wire interconnections is essential to maintaining the electrical integrity of an electronic device.
In addition to the protection of wire bonded connections, modern electronic circuits must be able to withstand a wide variety of environmental conditions, such as exposure to moisture, heat, mobile ion contaminants and abrasion. The search for suitable protection revealed several types of sealing or encapsulating materials, including a variety of flexible polymer compositions derived from polyurethanes, polysiloxanes, polyepoxides and other organics, plastics and the like. Such compositions are now well known for providing protective layers for circuits and attached semiconductor components. Despite improvement in performance of sealing and encapsulating materials, resistance to moisture, and ion penetration still remain as problems for most materials, while selected materials are subject to specific problems. For example, deficiencies of silicone-based encapsulants include poor solvent resistance and a relatively poor level of adhesion to substrate materials.
A review of encapsulant materials shows common use of silicone compositions. Despite the problems of solvent susceptibility and poor adhesion, these materials appear to offer a better balance of properties than other, previously used, types of material. U.S. Pat. No. 4,888,226 describes an electronic device encapsulated by a silicon dioxide-containing silicone resin gel that cures with a hard protective shell. The encapsulant material is an improvement over a silicone composition, in U.S. Pat. No. 4,565,562, which could fail when subject to thermal stress, abrasion or solvent attack. Non-silicone encapsulants in combination with silicone encapsulants provide performance improvement by contributing properties that silicones lack. An example of this is provided by U.S. Pat. No. 5,047,834 wherein one material gives protection around bonded leads of the lead frame while another provides environmental protection extending over the surface of the device and the leads. A silicone encapsulant provides environmental protection and covers a filled epoxy encapsulant that surrounds the bonded leads. The combined protection of silicone and epoxy materials overcomes the problems of each used individually.
The long term successful use of epoxy compositions as potting compounds and encapsulants for electrical industries may not apply directly in electronic packaging applications. Generally, epoxy resins alone are too rigid, brittle, moisture absorbing and susceptible to thermal shock to satisfy the requirements of bonded lead protection and electronic component encapsulation. The primary problem appears to be a lack of flexibility of cycloaliphatic polyepoxides, glycidyl ethers of bisphenol A and related epoxy resins. Evidence exists of numerous attempts to improve the flexibility of epoxy resins with the goal of developing suitable electronic encapsulant materials. One frequently addressed approach uses toughener addition to increase the flexibility of epoxy resin compositions. The toughened epoxy formulation may also include a reactive diluent. U.S. Pat. No. 5,726,216, for example, describes the addition of a toughener or flexibilizer to e.g. glycidyl ethers of various phenolic compounds. Materials as tougheners for such resins include oligomers of engineering thermoplastics which contain functional groups capable of reacting with the epoxy resin during curing. Suitable oligomers may be flexible molecules with terminal reactive substituents, such as epoxy groups, at one or both ends of a long, predominantly aliphatic hydrocarbon chain. Additional references, including U.S. Pat. Nos. 4,578,425; 4,778,851; 5,053,496; 5,420,202; 5,461,112; 5,499,409; Canadian published application CA 2,164,915, and European published application EP 0 565 206, provide further evidence of physical property modification to provide more flexible epoxy resins, either by addition of toughening components that rely on molecular structure or the introduction of core/shell particles into the epoxy resin composition. In all cases, preferred compositions contain less than 40% of toughener. U.S. Pat. 4,639,503 discusses the addition of a phenol adduct of a conjugated diolefin polymer to an epoxy resin produced by reaction of epichlorohydrin and bisphenol A or a novolak resin, alicyclic epoxy resins and epoxy resins derived from halogenated bisphenol A. Up to 300 parts of phenol adduct of a conjugated diolefin polymer may be added to 100 parts of an epoxy resin to provide encapsulant compositions which, upon curing, survive pressure cooker (125xc2x0 C.xc3x97200 hrs) and heat cycle (100 cycles between xe2x88x9240xc2x0 C. and 120xc2x0 C.) testing. The glass transition temperatures (Tg) of cured compositions are in the range of 60xc2x0 C. to 85xc2x0 C. probably due to the relatively high amount of epoxy resin in the composition.
Despite efforts, an encapsulant has yet to be found to meet the needs previously outlined and in addition survive the thermomechanical shock and outgassing testing required of today""s sophisticated electronic devices and related hardware. Towards this goal, it would be beneficial to provide a material capable of flowing about an electronic device, to encase it, with appropriate adhesion, electrical properties, thermal stability, moisture resistance, solvent resistance, and shock resistance after curing to yield an encapsulant hard enough to resist penetration but soft enough to cushion impact.
The invention provides novel encapsulant compositions containing major quantities of components unlike any of the polyurethane, polyepoxide and polysiloxane base resins previously investigated. One version of an encapsulant composition, according to the present invention, includes a linear hydrocarbon chain, usually in the form of a polyolefin oligomer providing a flexible backbone with reactive functionality attached to the oligomer chain. The polyolefin oligomer is hydrophobic, providing moisture resistance, while the linear hydrocarbon chain yields a flexible cured encapsulant. Hydrophobic materials generally do not adhere well to other materials. An adhesion promoter, added to a polyolefin oligomer provides a second component for a liquid oligomer composition having the characteristics required of an encapsulant material. Addition of the second component to the composition provides cured encapsulant structures with desirable characteristics of adhesion to selected substrates, and, depending on its other properties, an adhesion promoter may also add to thermal stability over extended periods of time. Other materials may be added to assist with compatibility of the main components and to produce cured compositions with the required modulus and coefficient of thermal expansion (CTE).
Some packaging applications require encapsulant formulations with extremely low viscosities before curing in order to penetrate confined spaces in the proximity of electronic components. In such applications, liquid oligomer compositions comprising liquid oligomer and an adhesion promoter also include a viscosity-modifying component.
Viscosity-modifying components used herein are different from low molecular weight reactive diluents known for use in encapsulants. Low molecular weight diluents become part of the covalent resin network upon curing. The disadvantage of use of such diluents appears when species associated with the diluent impart characteristics that may be detrimental to the cured resin network. For this reason, selection of viscosity-modifying components for the present invention emphasizes the need for viscosity adjustment without adverse impact upon the properties of the cured composition. Preferably, the product of curing an encapsulant composition retains the protective, repellent properties of the liquid oligomer, the adhesion building benefits of the adhesion promoter and optionally viscosity adjustment provided by a viscosity-modifying component. Thermal curing of the encapsulant composition produces an interpenetrating network of crosslinked polymers to hold the liquid oligomer, adhesion promoter and the viscosity-modifying component, if present, in a suitably condensed condition to prevent any undesirable emissions of low molecular weight contaminants. During formation of the interpenetrating network, the adhesion promoter and viscosity-modifying component enter different polymer systems. Either one may join with a liquid oligomer, for polymer generation, but not with each other. In most instances the generation of independent, cured polymer networks occurs without noticeable phase separation.
Preferred compositions of the current invention are low viscosity fluids that yield cured encapsulants having glass transition temperatures (Tg) below about 0xc2x0 C. The viscosities of the low viscosity fluids extend from about 1,000 cps to about 200,000 cps to allow the uncured encapsulant composition to flow into all of the spaces over, under and around the devices to be encapsulated. After curing, due to its low Tg, the encapsulant remains relatively soft and pliable and in optimum condition to cushion and protect delicate bonded leads from damage. Tg values for cured compositions, according to the present invention, indicate that the rubbery to glassy state transition occurs at temperatures much lower than any previously reported epoxy for electronic component encapsulants.
A variety of reactive groups, useful for the present invention, may be attached to the linear hydrocarbon chain. Such groups include epoxy groups, (meth)acrylate groups, carboxyl groups, hydroxyl groups, isocyanate groups, amine groups, allyl groups, phenolic groups, vinyl groups, vinyl ether groups, anhydride groups, alkyd groups, cyanate ester groups, silyl groups, mercapto groups, and the like.
Thermally curable encapsulant compositions, according to the present invention, show robust performance by surviving aggressive evaluation via high temperature and high humidity conditioning and thermal shock requirements of JEDEC 1 testing, pressure cooker testing, thermal cycling and high temperature storage.
More particularly, these thermomechanical-shock-resistant cured compositions for solventless, hydrophobic resin encapsulation of electronic components comprise a non-silicone oligomer including a flexible hydrocarbon backbone having reactive functionality, preferably terminal reactive functionality, an adhesion promoter; and optionally a curable viscosity-modifying component. The composition includes up to about 40% by weight of the adhesion promoter, and the cured composition has a glass transition temperature below 0xc2x0 C.
In cases where the cured encapsulant includes an interpenetrating polymer network, the invention provides a thermomechanical-shock-resistant cured composition for solventless, hydrophobic resin encapsulation of electronic components comprising a plurality of polymers forming the interpenetrating polymer. Interpenetrating network formation occurs by curing a liquid composition comprising a non-silicone oligomer, including a flexible hydrocarbon backbone with reactive functionality; an adhesion promoter; and a viscosity-modifying component. The liquid composition includes up to about 40% by weight of adhesion promoter. The adhesion promoter enters a different polymer of the interpenetrating network from the viscosity-modifying component and the cured composition has a glass transition temperature below 0xc2x0 C.
As used herein, the following terms have the defined meanings.
1. The term xe2x80x9cencapsulant compositionxe2x80x9d means the final uncured composition applied around an electronic component or device to encapsulate the component or device, protecting it from heat and environmental contamination.
2. The term xe2x80x9ccured encapsulantxe2x80x9d is the protective mass around an electronic component or device after curing an encapsulant composition.
3. The term xe2x80x9cliquid oligomer compositionxe2x80x9d means the composition containing a liquid polyolefin oligomer and an adhesion promoter with no viscosity-modifying component.
4. The term xe2x80x9cliquid oligomerxe2x80x9d means a prepolymer structure, having reactive groups attached to a flexible, essentially linear backbone containing between about 20 and about 700 carbon atoms and having a molecular weight from about 300 to about 10,000.
5. The term xe2x80x9cviscosity-modifying componentxe2x80x9d means a low molecular weight, low viscosity liquid sufficiently reactive to participate in a curing reaction to produce a crosslinked polymer network.
6. The term xe2x80x9c(meth)acrylatexe2x80x9d designates the use of either methacrylate functional species or acrylate functional species.
7. The terms xe2x80x9cglass transition temperaturexe2x80x9d and xe2x80x9cTg xe2x80x9d mean a property of a polymer, defined as the temperature at which the polymer changes between a rubbery and a glassy state without any change in phase.
8. The term xe2x80x9cthermal shock testingxe2x80x9d refers to the explosive production of bubbles or craters in the mass of a cured and preconditioned encapsulant when the temperature of the mass is rapidly raised from room temperature to at least 220xc2x0 C. by holding the encapsulant in contact with a hot-plate for a specified amount of time. Moisture in the encapsulant layer responds by expanding, vaporizing explosively to the extreme jump in temperature causing voids or vapor pockets in the coating. For this reason, samples that fail thermal shock testing are said to exhibit xe2x80x9cpopcorning.xe2x80x9d
The beneficial effects, for thermosetting encapsulants disclosed herein, will be further addressed in the following discussion of preferred compositions.